Glass-ceramic articles, i.e., articles prepared through the controlled heat treatment of precursor glass articles which results in the crystallization in situ thereof, originated in U.S. Pat. No. 2,920,971. As explained in that disclosure, the production of glass-ceramic articles contemplates three general steps. First, a glass forming batch of a proper composition, and commonly containing a nucleating agent, is compounded and melted. Second, the melt is simultaneously cooled to a temperature below the transformation range and a glass article of a desired geometry shaped therefrom. Third, the glass article is exposed to a predetermined heat treatment which effects crystallization in situ of the glass article. Customarily, the heat treatment is accomplished in two steps. Hence, the glass article is initially heated to a temperature somewhat above the transformation range to cause the generation of nuclei and incipient crystallization in the parent glass body. Thereafter, the nucleated article is heated to a higher temperature, frequently at or above the softening point of the glass, to promote the growth of crystals on the nuclei.
Because the crystals are essentially simultaneously developed upon a multitude of nuclei present throughout the precursor glass body, the glass-ceramic article conventionally consists of relatively uniformly-sized, fine-grained crystals homogeneously dispersed within a glass matrix, the crystal phase commonly constituting the predominant proportion of the article. U.S. Pat. No. 2,920,971 provides a much more complete review of the microstructure of glass-ceramic bodies and the manner in which they are made. Accordingly, reference is made to that patent for further discussion of those points. As can be understood therefrom, the crystal phases generated in glass-ceramic articles are dependent both upon the composition of the precursor glass and the heat treatment applied thereto.
Whereas the majority of the glass-ceramic products which have been prepared has exhibited an opaque appearance, various workers have described transparent, highly-crystalline, glass-ceramic articles. The crystal phases present in such articles have demonstrated very low birefringence, and/or indices of refraction substantially matching that of the residual glassy matrix, and/or extremely fine crystal sizes. The crystal phase most prevalent in transparent glass-ceramics has been termed beta-quartz solid solution. U.S. Pat. No. 3,252,811 is illustrative of such products and provides a rather detailed discussion of the composition and microstructure of beta-quartz solid solution crystals.
Because of the extensive crystallization present in glass-ceramic articles, the mechanical strengths demonstrated by those articles are customarily somewhat greater than those displayed by the parent glass body. Nevertheless, considerable research has been expended in developing means for improving the mechanical strengths of glass-ceramic articles and numerous schemes have been proposed for accomplishing that purpose.
Various ion exchange techniques are described in such U.S. Pat. Nos. as 3,585,053, 3,585,054, and 3,585,055. In certain instances (U.S. Pat. Nos. 3,585,053 and 3,585,055), the identity of the crystals in the surface of the glass-ceramic is unaffected by the ion exchange, but compressive stresses are set up in a surface layer on the article. In other instances, exemplified by U.S. Pat. No. 3,585,054, the identity of the surface crystals is changed, the new crystals having a lower coefficient of thermal expansion thereby creating an integral surface compression layer in the article. U.S. Pat. No. 3,637,453 discloses first exposing a glass-ceramic article to an ion exchange reaction and thereafter thermally tempering the body. U.S. Pat. No. 3,931,438 teaches the manufacture of strengthened glass-ceramic articles by forming a laminated composite body wherein the core portion has a different composition from that of the skin layers. The compositions are so chosen that, when the precursor or laminated glass body is crystallized, the skin layers densify to a lesser extent than the core. Surface compressive stresses are set up because of the greater volume shrinkage of the core glass-ceramic with respect to the glass-ceramic skin.
Articles demonstrating mechanical strengths superior to those exhibited by glass or glass-ceramic bodies have been prepared consisting of an internal or core portion of glass and a crystallized surface layer. U.S. Pat. No. 3,253,975 is illustrative of that practice and describes heat treating a glass body of a particular composition to generate a crystalline surface layer thereon, the crystals displaying a lower coefficient of thermal expansion than the glass. This established a uniform compressive stress in the surface layer which led to a significant increase in the mechanical strength of the body.
Each of those methods is limited to specific composition areas for which each is operable. Moreover, the patented methods involve complex machinations and expensive equipment, e.g., baths of molten salts for ion exchange reactions, and/or very carefully controlled heat treatments to generate the desired surface crystallization, and/or complicated forming techniques to produce laminated articles. Accordingly, it would be highly desirable to devise a rapid, relatively inexpensive method for strengthening glass-ceramic and surface-crystallized glass articles.
U.S. Pat. No. 1,973,501 discloses the strengthening of soda lime glass articles by exposing such to temperatures at or somewhat below the softening point of the glass in an atmosphere containing vapors of sulfur dioxide (SO.sub.2) or sulfur trioxide (SO.sub.3). Those vapors react with the Na.sub.2 O in the glass surface to form Na.sub.2 SO.sub.3 or Na.sub.2 SO.sub.4. Those compounds are stated to flux the glass surface slightly so as to round off the minute flaws inherently present in the glass surface. This practice improves the weathering resistance and mechanical strength of the glass.